Filtering and bypass capacitors are commonly used in high speed logic circuitry to reduce EMI emissions and internal noise and to improve immunity. Filtering performance is degraded at high frequencies by coupling (mutual inductance) between the input and output sides of the filter on the printed circuit board (PCB) layout. Known solutions include using two parallel capacitors and appropriately spacing the capacitors, typically, by placing them on opposite sides of a trace on one side of the PCB, or by separating them by a small distance along a trace on one side of the PCB.
Typical inexpensive low pass filters consist of a single capacitor. FIG. 1A exemplifies such a circuit, and shows a capacitor 103 of capacitance C connected between a signal line 101 and a ground line 102. The single-capacitor filter is only effective up to a few hundred megahertz, due to the mutual inductance that results from the current flowing through the capacitor coupling magnetic flux from one side of the filter 104 to the other 105. At higher frequencies, the impact of this mutual inductance causes attenuation to increase significantly, thus limiting the effectiveness of the filter.
FIG. 2A shows a perspective view of a multi-layer PCB 201 with a one-capacitor low-pass filter. The capacitor 202 is connected on one side to a signal trace 203 (that runs between two ports 204) and on the other to a via (through hole) 205 that buries through the layers of the PCB to connect to a lower ground layer.
FIG. 2B shows a top view of the same PCB and one-capacitor low-pass filter as in FIG. 2A.
A known alternative to single-capacitor filters is to employ two capacitors connected in parallel. Two-capacitor low pass filters are still low cost and a more effective alternative at high frequencies than single capacitor filters. FIG. 3 shows an exemplary circuit diagram. The capacitors 303 and 304 are each of half the capacitance of the single-capacitor case shown in FIG. 1 (103) and are connected, on the same side of the PCB, in parallel between a signal line 301 and a ground line 302. In the two-capacitor case, there are three associated inductance loops: an input loop 305 with self-inductance L1, a middle loop 306 with self-inductance L2, and an output loop 307 with self-inductance L3. In addition, mutual inductances between the input and output loops (M13) and input and middle loops (M12) are also present and serve to further reduce performance at high frequencies.
FIG. 4 shows a top view of a PCB layout with a two-capacitor low-pass filter where the capacitors 402 & 403 are connected on the same side of the PCB and also on the same side of the trace 401 (a prior art layout) and to their respective ground vias 404 & 405.
FIG. 5 is a top view of a PCB with a two-capacitor low-pass filter layout where the two capacitors 502 & 503 are connected on opposite sides of the signal trace 501 (another prior art layout) but on the same aside of the PCB.
It has been found that increasing the physical separation between the two capacitors leads to an overall reduction in the mutual inductance and thus to enhanced performance. FIG. 6 exemplifies a prior art solution, showing a top view of a PCB where the two capacitors 602 & 603 are connected on the same side of the PCB and on the same side of the trace 601 but are separated by a distance 605, d=6 mm.